Account for use of reference_date property

[wtbarnes]

This PR includes a few fixes to account for the existence of the new reference_dateproperty on Map which is the correct date to use when constructing the coordinate frame and observer coordinate.

This also now assumes the existence of the "T_OBS" key in the metadata since this is what is used for constructing reference_date in the case of an AIA map.

Since reference_date was only introduced in sunpy v6 we should decide how to go about using this property or just make the minimum version of sunpy v6.

See also sunpy/sunpy#7810

Summary by Sourcery

Update the code to use the 'reference_date' property for constructing coordinate frames and observer coordinates, assuming the presence of 'T_OBS' in metadata for AIA maps. Adjust tests to reflect these changes and improve accuracy.

Bug Fixes:

• Fix the use of the incorrect date for constructing the coordinate frame and observer coordinate by using the new 'reference_date' property instead of 'smap.date'.

Enhancements:

• Assume the existence of the 'T_OBS' key in the metadata for constructing 'reference_date' in AIA maps, aligning with changes in sunpy v6.

Tests:

• Update tests to use 'u.allclose' for asserting observer coordinate values with a relative tolerance, improving test accuracy.
• Remove the test that checks for a warning when 'T_OBS' is missing from metadata, as this scenario is no longer applicable.

[nabobalis]

I will leave it up to you @wtbarnes when you want to merge.